CRYPTOGRAPHY AND NETWORK SECURITY -3IAE ANSWER KEY
SNS COLLEGE OF ENGINEERING
Kurumbapalayam(Po), Coimbatore – 641 107
Accredited by NAAC-UGC with ‘A’ Grade
Approved by AICTE & Affiliated to Anna University, Chennai
INTERNAL ASSESMENT EXAMINATIONS - III
COURSE: B.E – CSE
IT2352 – CRYPTOGRAPHY AND NETWORK SECURITY
ANSWER KEY
PART-A
1.Give an example each for substitution and transposition ciphers
Additive cipher in which the shifting of letter is been done by giving the value as key value.
when the key value is given the shifting of letter is done according to the given term.
In transposition cipher it rearranges the character in the plain text to form the cipher text. The
railfence is writing the plaintext in two rows proceeding down then across and reading the cipher
text, then down.
2. Differentiate between active attack and passive attack.
PASSIVE ATTACK
 In passive it only monitors the
transmission
 Very difficult to detect
 The attack ismainly on the prevention
method
 It does not affect the system
ACTIVE ATTACK
It involves in the modifying of the data stream
Easy to detect
It is quite difficult to prevent
It affects the system
3. What are the modes of DES?
ECB(electronic code book)
o CBC(cipher block chaining)
o CFB(cipher feedback)
o Output feedback
4. Define factoring?
It is the decomposition of an object into a product of other objects or factors, which when
multiplied together give the original
5. What is discrete logarithm problem?
CRYPTOGRAPHY AND NETWORK SECURITY -3IAE ANSWER KEY
It defined with regard to multiplicative cyclic groups. if G is a multiplicative cyclic group and g
is a generator G,then from the definition in element h in G can be written as g is the power of x
,x has some value.
6. Write differences between MD4 and Secure Hash Algorithm
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MD4
Speed is faster
It uses little endian scheme
No limit on message size
SECURE HASH
Not that much speed
It uses big endian scheme
Limit on message size
7. List out the services provided by PGP.
Digital signature, message encryption, compression, email compatibility and segmentation.
8. State the use of tunnel mode in IP Security
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It provides the protection to IP packet
It is used when one or bothends of an SA is a security gateway
9. Name three different classes of intruders and also define intruders
Masquerader ,misfeasor, clandestine user
10. Define firewall and mention the types of firewall
It is inserted between the internet and LAN for security purpose.it protects the LAN from
internet based attacks. types are Packet filtering router, Application level gateway, Circuit level
gateway
PART B
11a)i) State and explain chinese remainder therorem with example
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Find a number x such it have remainder of 1,A ->(a1,a2,a3,……ak),where Aelement of
zmi and ai=Amod mifor 1<=i<=k.suppose thatm1,m2…..mi ae pairwise relatively prime
positive integers and let a1,a2….ar be integers .the system of congruences x=a i(mod
mi)for the given values has a unique solution
Then x0 and x1 is equals 0(mod Mi)
The value of modulo m is been determined
CRYPTOGRAPHY AND NETWORK SECURITY -3IAE ANSWER KEY
a)ii) State fermat’s and euler theorem.
if p is prime and is an integer not divisible by p then,
ap-1=1(modp)
ap=a(modp)
euler form:
api(1 mod n)
R={x1,x2,xpi(n))}
11)b) Discuss classical crypto system and its types
Shift cipher, playfair cipher, hill cipher, one time pad, monoalphabetic cipher, polyalphabetic
cipher, vigenre, transposition cipher
12)a) Explain about DES in detail.
Details of single round, key generation,DES encryption,DES decryption,DES weak
keys,advantage of DES and disadvantage,double DES
12b)i) Explain RSA algorithm.
Algorithm
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Pick two large prime number p and q p!=q
Calculate n=p*q
Calculate pi(n)=(p-1)(q-1)
Gcd(e,pi(n))=1
Calculate d so that d.e(mod pi(n)=1 d is found
Ku={e,n}
KR={d,n}
C=Pemodn
P=Cd mod n
CRYPTOGRAPHY AND NETWORK SECURITY -3IAE ANSWER KEY
b)ii) Demonstrate the encryption and decryption for the RSA algorithm parameters
p=3,q=11,e=7,m=5. D=?
p=3,q=11,e=7,m=5
sol:
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n=p*q=3*11=33
pi(n)= (p-1)(q-1) =2*10=20
e=7,m=5 d=[3*7]mod pi(n)=1
d=3
C=Pe mod n =57 mod 33=14
P=Cd mod n = 143 mod 33 = 5
13a)i ) Briefly explain secure hash algorithm
SHA
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Features of SHA,secure hash algorithm(SHA 512),description of SHA,block cipher
text,SHA 512 logic
Append padding bits,append length,initialize of buffer,process message in 1024bit
block,SHA round function
13)b)i) Write about Digital signature standard
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It is of public key algorithm
Attacks,confidentiality
Encrypt the entire message with the sender secret key
Digital signature standardit also uses the secure hash algorithm and the DSA term used
over in it
b)ii) Elaborate digital signature algorithm.
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h(p-1)/q modp
it selects the private key and generate the a public key.the private key x must be a number
from 1 to (q-1)
public key is calculated from the private key as y=gx mod p
public key components(p,q,g)
CRYPTOGRAPHY AND NETWORK SECURITY -3IAE ANSWER KEY
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users private key of the messageH(M)
an additional integer(K)
14)a)i) Write short notes about
Kerberos.
Authentication protocol. The mutual authentication is been achieved in it. Kerberos terminology
Authentication server(AS), client, credentials, credentials cache, crypt hash, key, realm, ticket
granting system,ticket granting ticket
Kerberos server, client, application server
a)ii)X-509.
Directory service which maintains the data base
Version, certificate serial number, signature algorithm, issuer name, period of validity, subject
name, subject unique identifier, subject unique identifier, extension, signature
Authentication procedures
One way authentication, two way authentication, three way authentication
14)b)i) Discuss Pretty Good Privacy.
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PGP operation,
cryptographic keys and key Rings,
message format,
PGP message generation,
PGP message reception,
concept of trust
b)ii) Describe shortly SSL and SET.
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services providedby SET,
requirement for SET
features for SET,
CRYPTOGRAPHY AND NETWORK SECURITY -3IAE ANSWER KEY
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SET participants,
key technologies of SET
15)a)i) Explain intrusion detection system
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Anomaly detection,
signature base detection,
threshold detection,
audit record format,
components of IDS host agent module,
LAN module,
central module
a)ii) Briefly mention about viruses and its threats
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Phases of virus: dormant virus,
propagation,
triggering,
execution.types:
Parasitic, bootsector virus, stealth virus, polymorphic virus, metamorphic.
15)b) Write in detail about characteristics, types and limitations of firewalls
It is inserted between the internet and LAN for security purpose.it protects the LAN frominternet
basedattacks.types are
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Packet filtering router,
Application level gateway,
Circuit level gateway
CRYPTOGRAPHY AND NETWORK SECURITY -3IAE ANSWER KEY